Conventional FM transmitters using phase-locked loops (PLLs) to generate a high frequency carrier signal having its frequency synchronized by a substantially lower frequency input reference signal are well-known in the art. An inherent characteristic of a PLL is a low-pass filter in the loop that allows the loop to track long-term, slow variation of the frequency of the input reference signal, but prevents tracking high frequency changes, such as jitter, in the input reference signal. From the point of view of modulating the output, i.e., voltage controlled oscillator (VCO), side of the PLL, the loop characteristics permit high frequency modulation of the output, e.g., voice modulation, but prevent low frequency or DC modulation such as required to transmit asymmetrical data or to permanently offset the carrier frequency. To do both high and low frequency modulation, conventional transmitters have used a two-port modulation technique to simultaneously apply a modulating signal to both the PLL VCO and to a reference modulator for modulating the input reference signal.
Conventional reference modulators have used a pulse addition technique to increase the input reference frequency and a pulse subtraction technique to lower the input reference frequency. Because the required amount of modulation was a small fraction of the carrier frequency, the rate of pulse addition or subtraction was very low, e.g., sixty Hz. The very low pulse rate caused very low frequency spurious signals that had to be removed by the low-pass filter of the PLL to prevent the spurious signals from modulating the carrier. Adequate attenuation of the very low frequency spurious signals required an extremely low frequency cut-off, e.g., five Hz, for the low-pass filter of the PLL. Unfortunately, a five Hz PLL low-pass filter produced a PLL lock time that was too long to meet requirements for a frequency agile transmitter, i.e., a transmitter capable of being switched quickly to different pre-determined frequencies on demand.
To overcome the problem of an overly long lock time, manufacturers of conventional transmitters constructed the transmitters using two PLLs--one PLL having a very low cut-off frequency for filtering the very low frequency spurious signals and the other PLL having a moderate cut-off frequency for providing a sufficiently fast lock time when changing to a new frequency. While this dual PLL approach solved the lock time problem and provided DC and low frequency modulation capability, the approach was a costly one, because it doubled the number of PLLs and the space required therefor.
Thus, what is needed is a way of providing DC and low frequency modulation in an FM transmitter without a resultant overly slow PLL lock time and without the requirement for two costly PLLs.